This is a U.S. National Phase Application Under 35 USC 371 and applicant herewith claims the benefit of priority of PCT/SE00/00465 filed Mar. 9, 2000, which was published under PCT Article 21(2) in English, and Application No. 9900847-6 filed on Mar. 10, 1999 in Sweden.
The present invention relates to a multi-blade rotor designed to be used for energy conversion. The rotor could, for example, be used to extract rotional energy from a fluid flow, or, conversely, to produce a fluid flow from rotational energy.
It is particularly advantageous to connect the rotor to a generator and to extract energy from wind or water power.
Numerous rotors of the above kind are already known. Particularly common are different kinds of propellers having several propeller blades disposed on a shaft. In such propellers, each propeller blade forms an elongate surface, which extends essentially radially from the shaft and which is bent in its longitudinal direction.
Prior-art rotors do, however, suffer from several drawbacks, the most serious ones of which will be discussed in the following.
Firstly, the efficiency of traditional rotors is comparatively low. In the case of wind power, experts speak in terms of actual efficiency from a wind exposed area of one square meter of less than 1 Watt per m/s. A propeller having an exposed area of one square meter (a radius of about 60 cm), thus could extract about 10 Watt when the wind force amounts to 10 m/s.
Secondly, considerable forces are created axially, when prior-art rotors are exposed to the effects of a flowing fluid. In wind power stations, experts reckon that the supporting structure must be dimensioned to withstand twice the force that the rotor is capable of converting into energy.
Another problem found in prior-art rotors is that they generate a high-pitch sound that some individuals experience as an unpleasant noise.
A first object of the present invention is to provide efficient extraction of energy, such as wind power.
A second object of the invention is to provide a rotor that is inexpensive and simple to manufacture.
A third object of the invention is to provide a rotor that is relatively noiseless in operation.
These objects are achieved by means of a rotor of the kind outlined above.
In order to deflect the flow as desired, the rotor comprises several blades, each one of which consists of a single-curved surface, the leading edge of which extends essentially radially from the axis of rotation and is located in a plane forming a small angle (preferably 80-100 degrees) with the axis of rotation.
Further, each blade is mounted on a shaft, which extends in a plane normal to the axis of rotation and which is parallel to and spaced a distance from an imaginary radius extending outwards from the axis of rotation.
Each blade could form an essentially rectangular surface the curvature of which is such that the plane projection of the blade, i.e. the extension of the blade to the flow, forms an essentially square surface.
With rotor blades thus shaped the entire flow is deflected in a plane normal to the axis of rotation as seen in the extension of the blade. Since the leading edge projects essentially radially from the axis of rotation and the flow, after its deflection, is directed at right angles to the leading edge, the flow with respect to each blade occupies a 90xc2x0 sector area around the axis of rotation. If the number of blades were to exceed four, the deflected flow would be brought into contact with an adjacent blade, and consequently would brake the rotor. For this reason, the rotor in accordance with the invention comprises a maximum of four rotor blades.
In prior-art rotors the pivot shaft of each rotor blade, i.e. the shaft upon which each blade pivots, is arranged radially outwards from the axis of rotation, the leading rotor-blade edge being parallel to and spaced a distance from an imaginary radius. In accordance with the teachings of the present invention, the arrangement is exactly the opposite, i.e. it is instead the leading edge that is arranged to extend radially outwards from the axis of rotation and the pivot shaft that is parallel to and spaced from an imaginary radius. It is precisely this solution that generates the particular flow pattern that is described in the subject document.
Each blade could form an essentially rectangular surface, the curvature of which is the reason why the plane projection of the blade, viz the extension/expanse that the blade presents towards the flow, forms an essentially square surface.
When a rotor in accordance with the invention is used to extract energy from a flow, such as an air flow, the rotor is arranged such that the flow streams in an axial direction and hits the leading edge of the blade first. Each blade thus is effective in forcing the flow to change its direction of movement, from essentially in parallel with the axis of rotation to a plane normal to the axis of rotation, essentially tangentially thereto. This redirection of the flow generates a force tangentially that gives rise to a torque force around the axis of rotation. A number of blades are arranged to cooperate, for which reason the entire rotor, when affected by a fluid flow, is caused to rotate.
In addition, the flow that moves past the trailing edge of each blade, attains comparatively high flow velocities, thus creating an ejector effect that brings along essentially stagnant fluid from the leeward side of the blade. In consequence, some negative pressure is generated behind the blade, which further increases the torque.
Since the entire flow is caused to change its direction, practically no air is allowed to pass through the rotor and behind the rotor, a wake is formed, which continuously is replenished with surrounding fluid.
In tests carried out under secrecy with a prototype of the inventive rotor in a wind stream, supply of smoke revealed a flow pattern approximately configured as shown in FIG. 1.
The structure and orientation of the blades and the flow image that they produce give remarkable power-extraction results. The test referred to above was carried out with an inventive prototype having a wind-receiving area of about one square meter, in environments where the wind force amounted to about 10 m/s. The effective power that could be extracted amounted to about 200 Watt, at a rotational speed of 140 rpm. The power thus obtained was highly surprising.
Furthermore, it was found that the rotor essentially obtains its full capacity immediately after start-up. The power that it is possible to extract thus is independent of the rotational speed.
The angle between an imaginary plane interconnecting the leading and trailing edges of the blade and the axis of rotation advantageously is in the range of 30-50xc2x0, and preferably it is about 45xc2x0.
In addition, the blades could be spring loaded for rotation about the pivot shaft for automatic adaptation to the flow velocity.